1. Field of the Invention
The present invention relates to an image forming apparatus that forms images using an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
In recent years, image forming apparatuses capable of forming color images (such as copiers and printers) have been required to form high-quality images on a plurality of types of sheets at high speed. Examples of methods to form images at high speed include a method that images are continuously formed by an image forming apparatus having image forming units for respective colors (yellow (Y), magenta (M), cyan (C), and black (K)).
Electrophotographic image forming apparatuses form electrostatic latent images by irradiating photosensitive drums with laser beams via a polygon mirror rotatively driven by a polygon motor, transfer toner images obtained by developing the electrostatic latent images using toner to an intermediate transfer belt, and further transfers the toner images to a sheet. Examples of methods to operate electrophotographic image forming apparatuses (color printers) at high speed include the following: a method that all processes (charging of photosensitive drums, exposure of the photosensitive drums with laser beams, development of electrostatic latent images using toner, transfer of toner images to a sheet, fixing of the toner images to the sheet, and so on) are speeded up, and a method that page intervals between images when images are formed in succession are reduced.
To form high-quality images using electrophotographic image forming apparatuses, a technique to align a position at which an image is formed with a sheet, a technique to reduce color shift in superimposing images of a plurality of colors on an intermediate transfer belt, and a technique to reduce changes in the densities of images to be formed.
In general, to reduce color shift caused by environmental changes such as temperature changes, aging changes, etc. of photosensitive drums provided for respective colors and an intermediate transfer belt, an electrophotographic image forming apparatus forms registration patches on the intermediate transfer belt, and reads the patches using a sensor to measure the amount of image shift. Then, based on the measurement result, the image forming apparatus corrects for the write timing of electrostatic latent images on the photosensitive drums in subsequent image formation. Also, the image forming apparatus provides control to form image density measurement patches on the intermediate transfer belt so as to make image densities uniform during successive image formation, and correct image densities according to densities obtained by reading the patches using a sensor so as to reduce changes in image densities caused by environmental changes.
While it is necessary to provide control to correct for various changes described above so as to form high-quality images, it is necessary to perform various correcting operations described above with little influence on image formation productivity so as to attain high productivity in image formation.
On the other hand, to efficiently perform and complete adjustment operations relating to color shift correction and toner density correction, there has been proposed a method that controls the order in which the adjustment operations are performed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2004-125988). According to the technique described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-125988, the adjustment operations performed by an image forming apparatus having a plurality of image formation speeds (process speeds) at which images are formed include an adjustment operation performed only at a reference speed, and adjustment operations performed at various speeds. When the adjustment operation performed only at the reference speed is to be performed, the speed is changed to the reference speed first, and then the adjustment operations that can be performed at various speeds are sequentially performed.
The image forming apparatus described above changes the image formation speed according to a sheet on which an image is to be formed. On this occasion, when various adjustment operations are to be performed at various image formation speeds, if an adjustment operation that cannot be performed at that image formation speed arises, speed change is required, which increases adjustment time. For this reason, most of the various adjustment operations can be performed at the various image formation speeds, but part thereof can be performed only at the reference speed. As a result, when adjustment is required while the image forming apparatus is operating at an image formation speed other than the reference speed, the speed is changed to perform the adjustment operation, and after that, the speed is changed again. Thus, adjustment takes long.